SUMMARY: 

General Lee was advancing towards Maryland when one of McClellan's soldiers happened to find Lee's battle plans which were wrapped around some cigars abandoned by one of Lee's soldiers. Knowing Lee's plans, McClellan had  a clear advantage to help him defeat them. Unfortunately, McClellan was not urgent enough in pursuing Lee's troops and missed teh oppurtunity to defeat them wholly. But McClellan's soldiers were able to use this to advance on Lee, forcing him to have to send for emergency help to come to Sharpsburg near the Antietam Creek. McClellan had a great plan but did not follow through with it fully; he attacked Lee's flanks but not in an efficient or very effective way. McClellan also thought that Lee's army outnumbered his own (which was not true at all) so he did not use all of his soldiers in the fear of losing them all and not having any for a surprise attack with soldiers Lee did not even hav. Despite that, this battle was one of the bloodiest battles in America. By the next day, Lee retreated and the Union had won.